U.S. Secretary of State John Kerry, left, shakes hands with Under Secretary-General for Legal Affairs Miguel Serpa Soares after signing the Arms Trade Treaty during the 68th session of the United Nations General Assembly at the U.N.

UNITED NATIONS — The United States, the world’s largest arms dealer, has joined more than 90 other nations in signing a treaty that regulates global arms trading, but there is strong resistance in the Senate, which must ratify it.

Secretary of State John Kerry, who signed the Arms Trade Treaty on Wednesday, said it was a “significant step” in keeping the world safe and preventing terrorists and others from obtaining conventional weapons.

The Obama administration’s move is seen as critical to the treaty’s success. The U.S. was the 91st country to sign, but the treaty will not take effect until 50 nations have ratified it. Only four had ratified the treaty as of Wednesday.

Many of the world’s other top arms exporters have yet to sign and opposition in the Senate, backed by the powerful National Rifle Association, means U.S. ratification will be difficult.

“This is about reducing the risks of international transfers of conventional arms that will be used to carry out the world’s worst crimes,” Kerry said.

He said it would require other countries to put in place the same arms export restrictions that the United States already has in force.

“This is about keeping Americans safe and keeping America strong, and this is about promoting international peace and global security,” he said.

Addressing U.S. critics of the treaty, the former senator said fears that it would undermine Americans’ constitutional right to keep and bear arms are not grounded in reality.

For one, the treaty does not regulate domestic weapons sales.

“This treaty will not diminish anyone’s freedom,” he said. “It recognizes the freedom of both individuals and states to obtain, possess, and use arms for legitimate purposes.”

Kerry said the U.S. “would never think about supporting a treaty that is inconsistent with the rights of Americans, the rights of American citizens to be able to exercise their guaranteed rights under our Constitution.”

Sen. Bob Corker of Tennessee, the top Republican on the Senate Foreign Relations Committee, is one of several conservative lawmakers who have concerns about the treaty, which he said raises “significant legislative and constitutional questions.”

In a letter sent Tuesday to President Barack Obama, Corker warned the administration against taking any steps to put the treaty in place unless and until the Senate ratifies it.

“The Senate has not yet provided its advice and consent, and may not provide such consent,” he wrote.

In addition to questions about the Second Amendment, Corker said many of the treaty’s provisions regarding arms sales will require congressional approval.

The treaty will require countries that ratify it to establish national regulations to control the transfer of conventional arms and components and to regulate arms brokers. It will not control the domestic use of weapons in any country.